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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2000 TRAVERWOOD ROAD

ANN ARBOR, MI 48105

 

May 4, 2012

 OFFICE OF

AIR AND RADIATION

MEMORANDUM

SUBJECT: NOX Emissions from DPF Regeneration

FROM: Lauren Steele, Office of Transportation and Air Quality

TO: Docket EPA-HQ-OAR-2011-1032

Estimates of Emissions of Oxides of Nitrogen from Regulated 

Emergency Vehicles 

The Heavy-Duty Highway Rule was projected to reduce oxides of nitrogen (NOX) 

exhaust emissions from the heavy-duty on-road fleet by 1.76 million tons in 2020, and by

2.48 million tons in 2030. In that rule’s final Regulatory Impact Analysis, EPA projected 

that in 2010 the controlled heavy-duty fleet would emit approximately 2 million tons of 

NOX emissions, decreasing to about 600,000 tons in 2020 and about 300,000 tons in 

2030, as a result of that rule.1For purposes of this memo, we are considering that rule’s 

control case as the current fleet baseline scenario.

As noted in Section VII.B of the proposed rule preamble, class 3 through 8 fire 

apparatus and ambulances represent about 0.8 percent of annual new heavy-duty truck 

registrations.2 If we assume these vehicles have the same emissions profile over time as 

other vehicles, we could estimate that their annual emissions collectively may also 

represent a comparable fraction of the fleet emissions in a given year. For the years 

projected in the RIA for the Heavy-Duty Highway Rule, this would indicate emergency 

vehicles may have emitted on the order of 16,000 tons of NOX in 2010, may emit about 

5,000 tons in 2020, and may emit about 2,000 tons in 2030.

 

1

 Final Regulatory Impact Analysis for the “2007 Heavy-Duty Highway Rule,” EPA420-R-00-026, 

December 2000, Chapter II Health and Welfare Concerns and Emissions Benefits, Table II.B-4, available 

at http://www.epa.gov/otaq/highway-diesel/regs/ria-ii.pdf

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ICCT, May 2009, “Heavy-Duty Vehicle Market Analysis: Vehicle Characteristics & Fuel Use, 

Manufacturer Market Shares.”
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However, we have information that these vehicles have longer than average useful 

lives, leading to a slower fleet turnover rate. We also have information that these 

vehicles have below average vehicle miles traveled, with a large fraction of their engine 

hours at low load or idle. While these two factors would affect the emissions inventory

in opposite directions, they are unlikely to cancel each other out. Because we do not have 

enough data to quantify how these factors affect the emission estimates, we will use the 

values presented above for purposes of the current analysis.

On a per-truck basis, we can estimate what the NOX exhaust emissions from a 

certified engine would be if it emitted NOX at the level of the standard, 0.2 gram per 

brake-horsepower-hour (g/bhp-hr). As an illustration, we have estimated what that might 

represent in terms of emissions from a truck operating at 200 horsepower over a typical

year with 1,200 operating hours. This would be representative of a 500-hp pumper truck 

with an average engine operating load of 40 percent of capacity. This illustration would 

yield a hypothetical fire truck that would emit about 100 pounds of NOX exhaust 

emissions in a year, or 0.05 ton per year.

These estimates are provided as a basis for comparison, as the following sections 

explore some different scenarios.

Estimates of NOX Emissions from Regeneration 

The estimates of NOX emissions for certified engines generally include emissions 

during periods when the diesel particulate filter (DPF) is regenerating. During both 

active automatic and parked manual regenerations, emission rates may increase for some 

pollutants, especially NOX when NOX-reducing aftertreatment devices are not present 

downstream of the DPF. The higher than normal temperatures during active regeneration 

with high rates of oxidation occurring across the catalyst create conditions conducive to 

NOX formation.

Because of the potential effect on emissions, engine manufacturers are allowed to 

provide adjustment factors that account for the excess emissions during DPF 

regeneration, as part of the certification process. During a series of certification tests, the 

High NOX Emission Factor (EFHNOX) represents the NOX emission rate during an active 

regeneration, and the DPF regeneration frequency represents the fraction of tests during 

which an automatic active regeneration occurs. Passive regenerations would not be 

expected to increase NOX emissions, thus they are not counted in the DPF regeneration 

frequency for testing purposes. Figure 1 illustrates the relationship between the EFHNOX

and DPF regeneration frequency, based on EPA certification data for MY 2008 engines 

over the transient test cycle.
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0

1

2

3

4

5

6

H

0% 2% 4% 6% 8% 10% igh N

OX Emission Rate (g/bhp -hr)

DPF Regeneration Frequency

2008 Transient EFHNOx with FreqDPF

Figure 1: High NOX Emission Factor and DPF Regeneration Frequency

As noted above, excess emissions from DPF regenerations are generally 

accounted for as part of the engine certification process. Thus, during DPF soot 

accumulation mode the NOX emissions may be lower than the standard for some engines, 

and we see a general inverse relationship between DPF regeneration frequency and the 

EFHNOX. We can also observe from Figure 1 that the NOX emission rate during a 

regeneration may be many times higher than the standard. 

The amount of excess NOX emissions due to DPF regeneration would generally

be expected to be small on a vehicle whose engine is equipped with downstream NOX

emissions control, such as selective catalytic reduction. EPA’s certification data for MY 

2011 engines indicate the EFHNOX are generally below 1.0 g/bhp-hr, with some outliers. 

DPF regeneration frequencies for MY 2011 engines do not differ substantially from those 

of MY 2008 engines. For more information on DPF regeneration frequencies, see the 

docket memo entitled Fuel Use with Dosing for DPF Regeneration, submitted to the 

docket for this rulemaking.

Estimates of Excess NOX Emissions from Emergency Vehicles with 

More Frequent Regenerations and Higher Emission Rates

As noted in the memorandum to the docket on fuel dosing, it is possible that some 

manufacturers will submit applications for AECD’s or field modifications for engines on 

emergency vehicles with liberalized parameters under which automatic active and/or 

parked manual regenerations may occur, potentially increasing the NOX emissions from 

those vehicles. It is also possible that manufacturers may submit requests for engine 

recalibrations that enable more frequent passive regeneration and rely less on active 

regeneration, potentially changing the NOX emissions in more complex ways. It is thus 

difficult to predict the effects this rule may have on the NOX emissions from emergency

vehicles. 
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As described in the preamble to the proposed rule, EPA has proposed that engines 

with approved emergency vehicle AECD’s would not need to include these excess 

emissions during certification. In other words, the engines would be tested with 

emergency vehicle AECD settings inactive. For purposes of this illustration, we assume 

the regeneration NOX in the base case is included in the certified emission level, and 

excess regeneration NOX due to the AECD would be above the certified emission level.

Without speculating what fraction of the emergency vehicle fleet may receive 

AECD’s or field modifications with NOX recalibrations, for this illustration we will focus 

on a per-truck emissions estimate. For this hypothetical truck, we are assuming that the 

current certified configuration (the base case) includes a DPF regeneration frequency

(FREQDPF) of 5 percent, a high NOX emission rate of 2.5 g/bhp/hr (EFHNOX during 

regeneration), and a low NOX emission rate of 0.2 g/bhp-hr (EFLNOX when not

regenerating). This example is purely hypothetical – EPA is not aware of an engine with 

these specific emissions characteristics. The alternate cases explore the emissions effects 

when the DPF regeneration frequency (FREQDPF) increases to 20 percent, and when the 

EFHNOX increases to 5.0 g/bhp-hr, while holding other parameters the same. 

The values presented in Table 1 have been calculated using the following formula 

for the average NOX emission rate (EFA):

EFA = (FREQDPF)(EFHNOX) + (1- FREQDPF)(EFLNOX)

Where

FREQDPF = Base 5% or Alternate 20%

EFHNOX = Base 2.5 g/bhp-hr or Alternate 5.0 g/bhp-hr

EFLNOX = 0.2 g/bhp-hr in all cases

To extrapolate the average emission rate (EFA) to an annual value, we are 

continuing with the example of a truck with a 500-hp engine operating at an average load

of 40%, for a working power of 200 hp and average of 1,200 engine operating hours in a 

year.

Table 1: Per-Truck NOX Emission Estimates by Regeneration Percent

Scenario EFA Pounds per 

Year

Tons Per 

Year

Base Case

0.32 170 0.08

Base FREQDPF

Increased EFHNOX to 5g/bhp-hr 0.44 230 0.12

Increased FREQDPF to 20 Percent

Base EFHNOX 0.66 350 0.17

Increased FREQDPF to 20 Percent

Increased EFHNOX to 5g/bhp-hr 1.16 610 0.31
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As shown in the table, if the both regeneration frequency and the high NOX

emission rate were to increase as described, the annual NOX emissions from this truck 

could increase fourfold. We believe this is a highly conservative estimate. As shown in 

Figure 1, these parameters have an inverse relationship, so it is highly unlikely that both 

parameters would be maximized in the same engine configuration.

We present below in Table 2 the emissions estimates using an alternate 

calculation method that is based on the regeneration interval. Similar to the method used 

in the docket memo entitled PM Emissions Impacts, we have estimated in the base case 

that a single regeneration event has a duration of 30 minutes, and the interval between 

regenerations is 25 engine hours. In the alternate cases we explore the emissions effects 

if the regeneration interval is reduced to 12 or 8 engine hours, and each case uses the 

same EFHNOX and EFLNOX as in the previous illustration.

Table 2: Per-Truck NOX Emission Estimates by Regeneration Interval

Regeneration 

Interval

Operating Time 

Regenerating

EFHNOX

(g/bhp-hr)

NOX Emissions 

(TPY)

25 hours 2.0% 2.5 0.06

25 hours 2.0% 5.0 0.08

12 hours 4.0% 2.5 0.08

12 hours 4.0% 5.0 0.10

8 hours 5.9% 2.5 0.09

8 hours 5.9% 5.0 0.13

The values in Table 2 indicate that if regenerations were to occur at 8 engine-hour 

intervals instead of 25 engine-hour intervals, with the EFHNOX emission rate doubling 

from 2.5 to 5 g/bhp-hr, then the annual NOX emissions from this hypothetical fire truck 

could double. 

All of the above examples assume that no additional NOX control occurs 

downstream of the DPF. However, the majority of the heavy-duty fleet of MY 2010 and 

later is equipped with selective catalytic reduction NOX aftertreatment systems. When 

active, this technology reduces NOX emissions dramatically, including any excess NOX

emissions due to DPF regeneration.